Silicon nitride, in the form of powder, comprises a promising engineering material which will find increasing use in high temperature structural applications. This use is due to the excellent thermal and mechanical properties which are possessed by this compound. Components which have been fabricated from this material will possess high mechanical strengths at elevated temperature, good thermal shock resistance, moderate thermal conductivities, a low coefficient of thermal expansion and chemical inertness. Silicon nitride components may be utilized as a replacement for metals in heat engine applications due to the ability of this compound to withstand higher temperatures and their lower densities. This ability to withstand higher temperature constitutes a desirable factor due to the fact that the efficiency of engines increases with temperature. Furthermore, a reduction in weight will assist in increasing fuel efficiency and, in the case of turbine rotors, improve the response time.
With the increasing interest of industry in the use of silicon nitride, an economical method is required to produce a high surface area, non-agglomerated, high purity powder with a narrow particle size distribution. Silicon nitride, in the past, has been prepared by several different methods; by reacting silicon and nitrogen; by the reduction and nitridation of silica; by the decomposition of silicon diimide; by the reaction of halides, and by the decomposition of appropriate polymers.
As an example of prior methods for preparing silicon nitride, U.S. Pat. No. 4,122,152 discloses the preparation of this product by the carbothermal reduction and nitridation of silica. The process is effected by nitriding a mixture of carbon and silica in a nitrogen atmosphere at an elevated temperature for a predetermined period of time. The resulting compound contained from about 87 to 93 wt. % of alpha silicon nitride. Another patent, namely U.S. Pat. No. 4,117,095, discloses a process similar in nature to that of the previous patent, differing only in the fact that a small amount of silicon was added to the carbon and silica mixture prior to treatment with a nitriding atmosphere comprising nitrogen, ammonia, nitrogen and hydrogen, or nitrogen and an inert gas. Another U.S. Pat. No. 4,405,589 discloses the preparation of silicon nitride by mixing a compound comprising a polymer obtained from a chloroalkyl silane or a chloroalkaryl silane with H.sub.n SiCl.sub.(4-n). Following this, the mixture is reacted with ammonia to form a product which is then heated to a temperature between 1200.degree. and 1700.degree. C. in an inert or reducing atmosphere to form silicon nitride.